Team:Dundee/Project/PP1Capacities

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           <h2><b>PP1 Capacities</b> </h2>
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           <h2><b>PP1 Packing</b> </h2>
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          <h2>Introduction</h2>
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           <p>Microcystin’s toxic action lies in its ability to bind to the human Protein Phosphatase 1 (PP1). Microcystin covalently binds to PP1 in a one-to-one manner. Therefore higher binding potentials are achieved through having larger amounts of PP1 in the bacterial chassis <i>E. coli and B. subtils</i>. <br><br>
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           <p>Microcystin’s toxic action lies in its ability to bind to and thus deactivate the human Protein Phosphatase-1 (PP1). Microcystin covalently binds to PP1 in a one-to-one relationship. Therefore, the larger the quantity of PP1 produced within a given chassis,  the greater the "mopping" potential .<br><br>
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The two options explored for our mop bacteria were to export PP1 to the periplasm of E. coli and onto the membrane surface of <i>B. subtilis</i>. We investigate the PP1 capacity of both chassis options using geometrical packing. This then allows us to determine which chassis has a higher microcystin binding potential.
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The two options were explored for our bacterial  mop:  (i) to export PP1 to the periplasm of <i>E. coli</i> and (ii) to export and then bind PP1 to  the membrane surface of <i>B. subtilis</i>. To provide a crude estimate of "maximum mop efficacy" i.e. maximal PP1-binding potential,  we investigated the packing properties  of each  chassis option.
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           <h2>Theory</h2>
           <h2>Theory</h2>
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           <p>We considered the volumes of the bacteria and PP1 and used a cube approximation that took into account volume which was wasted, in packing, by the spherical shape of the protein. For this model we assumed there were no other surface proteins and protein production was not limited by any factors.<br><br>
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           <p>When considering the maximum number of PP1 molecules that cold be  located into the periplasm of <i>E. coli</i> or onto the membrane surface of <i>B. subtilis</i>, consideration for wasted space must be taken into account. This wasted space arises due to the physical configuration of PP1.<br><br>
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Calculations show the maximum number of PP1 which can fit on the surface of B.subtilis is between 60 000 -70 000. From the average we can calculate that the number of bacterial mops required to clean a toxic level of microcystin in a litre of water is 1.40 x 10<sup>10</sup>.<br><br>
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Before beginning such analyses, assumptions must be made regarding the shape and form of the PP1 molecules. To best describe the protein shape, without unnecessary over complication, PP1 was approximated as a sphere with a diameter equal to the  width of the minimal osculatory cubic enclosure of the molecule. This approximation allows for the possibility of random orientations of the PP1 molecules and therefore gives an average result. The radius of PP1 is thus calculated as ~9.1nm. <br><br>
 +
To calculate the available volume within    <i>E. coli</i> or <i>B. subtilis</i> cells, both  chassis were  approximated as cylindrical bodies with hemispherical ends. Furthermore, the dimensions of <i>E. coli</i> and <i>B. subtilis</i> cells were taken as being approximately equivalent and were based upon the known dimensions of <i>E. coli</i>:  0.5 μm in width by 2 μm in length [2] .<br><br>
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In <i>E. coli</i>, PP1 which would bind microcystin is free-flowing in the periplasm. The volume of the periplasm is much greater than the surface of <i>B. subtilis</i>. Therefore <i>E. coli</i> has the capacitive potential to be a more efficient mop. The maximum number of PP1 which can be packed into the periplasm is between 150 000 -200 000. Consequently, less bacterial mops are required to clean the same level of microcystin: 0.52 x 10<sup>10</sup>.<br><br>
 
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<center><img src="https://static.igem.org/mediawiki/2013/d/df/Ball.jpg"></img></center><br><br>
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<p>The equations describing the volume of spheres of radius r and cylinder of radius r and height h are given by : </p>
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<center><img src="https://static.igem.org/mediawiki/2013/4/41/Spherewriting2.png"></img></center>
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<p>Making use of  these equations, the volume of an <i>E. coli</i> cell can be calculated as the volume of a cylinder of radius 0.25μm and height 1.5μm and the volume of a sphere of radius 0.25μm (the two hemispheres can be brought together and calculated as a single sphere).</p>
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<center><img src="https://static.igem.org/mediawiki/2013/thumb/f/fc/Dundee_Eqn4.png/800px-Dundee_Eqn4.png" style="width:650px"></img></center>
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<h2><i>E. coli</i></h2>
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<p>
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For <i>E. coli</i> with a periplasmic width of 21nm, the periplasm volume can be calculated by considering the cell as two parts.<br><br>
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<center><img src="https://static.igem.org/mediawiki/2013/c/c9/Doublemb.jpg"></img></center>
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To calculate the periplasmic volume of <i>E. coli</i>,  we  next needed to calculate the volume enclosed by the inner membrane. This is again  taken to be a  cylinder now  of radius 0.25-0.021 = 0.229 μm and  length of 1.5μm. The two  flanking hemispheres  in this case have a radius 0.229μm. Therefore the volume enclosed by the inner membrane is</p><br><br>
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<center><img src="https://static.igem.org/mediawiki/2013/thumb/a/a7/Dundee_Eqn5.png/800px-Dundee_Eqn5.png" style="width:650px"></img></center><br><br>
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<p>Therefore, the volume of the periplasm in <i>E. coli</i> is calculated to be:</p>
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<center><img src="https://static.igem.org/mediawiki/2013/thumb/c/c0/Dundee_Eqn6.png/800px-Dundee_Eqn6.png" style="width:650px"></img></center>
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<p>To determine the  maximum number of PP1 molecules that could be packed into the periplasm, we determined the volume of  a single PP1 molecule.  This volume can be taken as that of a sphere of diameter 9.1nm. However, when considering the volume  that a single molecule will occupy, the space in between molecules must also be considered. To take account of  this wasted space, a cubic approximation was made:</p>
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<center><img src="https://static.igem.org/mediawiki/2013/thumb/f/ff/Packing.jpg/600px-Packing.jpg" style="height:200px"></img></center><Br><br>
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Thus, the volume of a PP1 molecule is assumed to be:
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<center><img src="https://static.igem.org/mediawiki/2013/thumb/7/7c/Eqn7.png/800px-Eqn7.png" style="height:150px"></img></center>
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<p>Hence, the maximum number of PP1 molecules that could be packed into the periplasm of <i>E. coli</i>  can be approximated by:</p>
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<center><img src="https://static.igem.org/mediawiki/2013/thumb/1/12/Eqn8.png/800px-Eqn8.png" style="height:150px"></img></center>
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<h2><i>B. subtilis</i></h2>
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<p>When considering the maximal packing of PP1in <i>B. subtilis</i> we recall that it  is a Gram-positive cell and so the PP1 molecules must be adhered in a monolayer to the surface of the cell membrane. <br><br>
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In packing PP1 on the surface of the membrane, the number of PP1 molecules that can be placed “side-by-side” along the height of the membrane and around the circumference of the membrane was calculated.<br><br>
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</p>
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<center><img src="https://static.igem.org/mediawiki/2013/7/7a/Packingballs.jpg"></img></center>
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<p>The number of PP1 molecules able to be stacked along the length of the cylindrical centrepiece of the cell (excluding the hemispheres) and around the circumference of the cylinder top:</p>
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<center><img src="https://static.igem.org/mediawiki/2013/c/c0/Pp1Cycn.jpg" style="width:750px"></img></center>
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<p>Multiplying the above results gives the total number of PP1 molecules which can occupy the surface of the cylindrical midsection of the bacteria.</p><br><br>
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<center><img src="https://static.igem.org/mediawiki/2013/thumb/7/7c/Eqn11.png/800px-Eqn11.png" ></img></center><br><br>
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<p>The number of PP1 molecules that could occupy the hemispherical ends can be calculated by approximating the hemispherical ends as discs. This approximation is appropriate due to the smaller contribution at these ends.</p>
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<center><img src="https://static.igem.org/mediawiki/2013/thumb/2/26/Eqn12.png/800px-Eqn12.png" style="width:490px"></img></center><br><br>
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<P>Thus, the total number of PP1 that could occupy the surface of <i>B. subtilis</i> can be calculated as the sum of the above.</p><br>
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 +
<center><img src="https://static.igem.org/mediawiki/2013/thumb/e/e9/Eqn13.png/800px-Eqn13.png" style="width:550px"></img></center>
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             <h2>Results</h2>
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             <h2>Conclusion</h2>
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              <p><i>E. coli</i> can store a significantly larger amount of PP1 than <i>B. subtilis</i>. This is because the volume of the periplasm in <i>E. coli</i>  is much greater than the available surface volume of <i>B.subtilis</i>.<br><br>
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              The maximum number of PP1 which can fit onto the surface of <i>B. subtilis</i> is between 60,000 -70,000, whereas the maximum number of PP1 which can be packed into the periplasm of <i>E. coli</i> is between 150,000 -200,000.<br><br>
+
Our calculations show the maximum number of PP1 molecules that could  fit on the surface of <i>B. subtilis</i> is approximately 35,000.  For  <i>E. coli</i>, the PP1 is contained within the periplasm. This periplasmic volume could potentially contain many more PP1 molecules (83 000)  than could be anchored to the membrane of <i>B. subtilis</i>. Therefore, <i>E. coli</i> has  the greater potential to be a more efficient mop. Consequently, using <i>E. coli</i>  would mean that  less bacterial mops would  be required to remove  a given quantity of microcystin. Using  this approximation, we  calculated that the number of  <i>E. coli</i> mops required to clean a toxic level of microcystin in a litre of water  to be 2.2x10<sup>9</sup> cells.<br><br>
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              Consequently the <i>E. coli</i> chassis has a greater PP1 capacity and higher microcystin binding potential. Therefore using <i>E. coli</i> requires fewer cells to mop up a fixed concentration of microcystin, making it a more efficient mop. .<br><br>
+
Having shown that <i>E. coli</i> has the potential to be the more efficient mop, this result was presented to the wet team in Week 1.  The wet lab continued to explore the use of both species, but  later in the project it was found that there were issues in exporting PP1 to the surface of <i>B. subtilis</i>. Hence our team  focussed on <i>E. coli</i> as the chassis for the bacterial mop, in line with the original  modelling prediction.</p>
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              As a result we focussed the wet team’s time and efforts more on developing the <i>E. coli</i> mop to form the prototype ToxiMop.</p>
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<h2>References</h2>
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<ul style="padding-left:15px">
 +
<li>[1] What is the hydrodynamic radius (RH)? Accessed: 18/09/2013: <br><a href="http://ecoserver.imbb.forth.gr/pdf/hydrodynamic_radius.pdf">http://ecoserver.imbb.forth.gr/pdf/hydrodynamic_radius.pdf</a></li><br>
 +
<li> [2] <i>Escherichia coli</i>, PortEco. Accessed: 18/09/2013: <br><a href="http://ecoliwiki.net/colipedia/index.php/Escherichia_coli">http://ecoliwiki.net/colipedia/index.php/Escherichia_coli</a></li><br>
 +
<li> [3] Width of periplasm, BioNumbers. Accessed: 18/09/2013: <br><a href="http://bionumbers.hms.harvard.edu/bionumber.aspx?&id=105387&ver=2">http://bionumbers.hms.harvard.edu/bionumber.aspx?&id=105387&ver=2</a></li><br>
 +
<li> [4] Protein serine/threonine phosphatase-1 (alpha isoform, type 1) complexed with microcystin-LR toxin, RCSB Protein Data Bank. Accessed: 18/09/2013: <a href="http://www.rcsb.org/pdb/explore/explore.do?structureId=1FJM#">http://www.rcsb.org/pdb/explore/explore.do?structureId=1FJM#</a></li><br>
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</il>
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Latest revision as of 19:07, 23 October 2013

iGEM Dundee 2013 · ToxiMop

Microcystin’s toxic action lies in its ability to bind to and thus deactivate the human Protein Phosphatase-1 (PP1). Microcystin covalently binds to PP1 in a one-to-one relationship. Therefore, the larger the quantity of PP1 produced within a given chassis, the greater the "mopping" potential .

The two options were explored for our bacterial mop: (i) to export PP1 to the periplasm of E. coli and (ii) to export and then bind PP1 to the membrane surface of B. subtilis. To provide a crude estimate of "maximum mop efficacy" i.e. maximal PP1-binding potential, we investigated the packing properties of each chassis option.

Theory

When considering the maximum number of PP1 molecules that cold be located into the periplasm of E. coli or onto the membrane surface of B. subtilis, consideration for wasted space must be taken into account. This wasted space arises due to the physical configuration of PP1.

Before beginning such analyses, assumptions must be made regarding the shape and form of the PP1 molecules. To best describe the protein shape, without unnecessary over complication, PP1 was approximated as a sphere with a diameter equal to the width of the minimal osculatory cubic enclosure of the molecule. This approximation allows for the possibility of random orientations of the PP1 molecules and therefore gives an average result. The radius of PP1 is thus calculated as ~9.1nm.

To calculate the available volume within E. coli or B. subtilis cells, both chassis were approximated as cylindrical bodies with hemispherical ends. Furthermore, the dimensions of E. coli and B. subtilis cells were taken as being approximately equivalent and were based upon the known dimensions of E. coli: 0.5 μm in width by 2 μm in length [2] .



The equations describing the volume of spheres of radius r and cylinder of radius r and height h are given by :

Making use of these equations, the volume of an E. coli cell can be calculated as the volume of a cylinder of radius 0.25μm and height 1.5μm and the volume of a sphere of radius 0.25μm (the two hemispheres can be brought together and calculated as a single sphere).

E. coli

For E. coli with a periplasmic width of 21nm, the periplasm volume can be calculated by considering the cell as two parts.

To calculate the periplasmic volume of E. coli, we next needed to calculate the volume enclosed by the inner membrane. This is again taken to be a cylinder now of radius 0.25-0.021 = 0.229 μm and length of 1.5μm. The two flanking hemispheres in this case have a radius 0.229μm. Therefore the volume enclosed by the inner membrane is





Therefore, the volume of the periplasm in E. coli is calculated to be:

To determine the maximum number of PP1 molecules that could be packed into the periplasm, we determined the volume of a single PP1 molecule. This volume can be taken as that of a sphere of diameter 9.1nm. However, when considering the volume that a single molecule will occupy, the space in between molecules must also be considered. To take account of this wasted space, a cubic approximation was made:



Thus, the volume of a PP1 molecule is assumed to be:

Hence, the maximum number of PP1 molecules that could be packed into the periplasm of E. coli can be approximated by:

B. subtilis

When considering the maximal packing of PP1in B. subtilis we recall that it is a Gram-positive cell and so the PP1 molecules must be adhered in a monolayer to the surface of the cell membrane.

In packing PP1 on the surface of the membrane, the number of PP1 molecules that can be placed “side-by-side” along the height of the membrane and around the circumference of the membrane was calculated.

The number of PP1 molecules able to be stacked along the length of the cylindrical centrepiece of the cell (excluding the hemispheres) and around the circumference of the cylinder top:

Multiplying the above results gives the total number of PP1 molecules which can occupy the surface of the cylindrical midsection of the bacteria.





The number of PP1 molecules that could occupy the hemispherical ends can be calculated by approximating the hemispherical ends as discs. This approximation is appropriate due to the smaller contribution at these ends.



Thus, the total number of PP1 that could occupy the surface of B. subtilis can be calculated as the sum of the above.


Conclusion

Our calculations show the maximum number of PP1 molecules that could fit on the surface of B. subtilis is approximately 35,000. For E. coli, the PP1 is contained within the periplasm. This periplasmic volume could potentially contain many more PP1 molecules (83 000) than could be anchored to the membrane of B. subtilis. Therefore, E. coli has the greater potential to be a more efficient mop. Consequently, using E. coli would mean that less bacterial mops would be required to remove a given quantity of microcystin. Using this approximation, we calculated that the number of E. coli mops required to clean a toxic level of microcystin in a litre of water to be 2.2x109 cells.

Having shown that E. coli has the potential to be the more efficient mop, this result was presented to the wet team in Week 1. The wet lab continued to explore the use of both species, but later in the project it was found that there were issues in exporting PP1 to the surface of B. subtilis. Hence our team focussed on E. coli as the chassis for the bacterial mop, in line with the original modelling prediction.

References